Trouble animating a mask...

I feel like this should be simple. I am making a video to showcase a new Android app. I've taken a generic Android phone and superimposed a screenshot of the app itself over the phone's screen. The app screenshot is too large (tall) to fit neatly over the phone's screen, so I applied an image mask. This works fine.

What I'd now like to do is "scroll" the image on the screen, to give it some motion, and so viewers can see more of the screenshot. I've tried animating the screenshot, but the mask moves with it. So how do I keep the mask stationary over the phone's screen while animating the screenshot?